A Lie Algebroid on the Wiener Space
Abstract
We define a Lie algebroid on the space of smooth 1-forms in the Nualart-Pardoux sense on the Wiener space associated to the stochastic linear Poisson structure on the Wiener space defined Léandre (2009).
1. Introduction
Infinite dimensional Poisson structures play a big role in the theory of infinite dimensional Lie algebras [1], in the theory of integrable system [2], and in field theory [3]. But for instance, in [2], the test functional space where the hydrodynamic Poisson structure acts continuously is not conveniently defined. In [4, 5] we have defined such a test functional space in the case of a linear Poisson bracket of hydrodynamic type. On the other hand, it is very well known [6] that the theories of Lie groupoids and Lie algebroids play a key role in Poisson geometry. It is interesting to study a Lie algebroid for the Poisson structure [4] defined analytically in the framework of [4]. We postpone until later the study the Lie groupoid associated to the same Poisson structure but in the algebraic framework of [5]. The definition of this Lie groupoid in the framework of [4] presents, namely, some difficulties. Moreover some deformation quantizations for symplectic structures in infinite dimensional analysis were recently performed (see the review of Léandre [7] on that). The theory of groupoids is related [8] to Kontsevich deformation quantization [9].
Let us recall what a Lie algebroid is [6, 10–13]. We consider a bundle E on a smooth finite dimensional manifold M. TM is the tangent bundle of M. Γ∞(E) and Γ∞(TM) denote the space of smooth section of E and TM. A Lie algebroid on E is given by the following data.
- (i)
A Lie bracket structure [·, ·] E on Γ∞(E) has in particular to satisfy the Jacobi relation
() - (ii)
A smooth fiberwise linear map ρE, called the anchor map, from E into TM satisfies the relation
()for any smooth sections X, Y of E and any element f of C∞(M), the space of smooth functions on M.
- (i)
The Bracket is defined by
()where L is the usual Lie derivative of a 1-form: if X is a vector field and α a k-form, then LXα is given by the Cartan formula() - (ii)
The anchor map is the map .
Infinite dimensional symplectic structures and their related Poisson structure were introduced by Dito and Léandre [19], Léandre [7, 20–22], and Léandre and Obame [23] in the infinite dimensional analysis, motivated by the theory of deformation quantization in infinite dimension. We refer to the review of Léandre on that in [7].
The goal of infinite dimensional analysis is to give a rigorous meaning to some formal considerations of mathematical physics. The formal operations of mathematical physics are defined consistently on some functional spaces. It is very well known, for instance, that the vacuum expectation of some operator algebras [25] is given by formal path integrals on the fields. Infinite dimensional analysis deals in the simplest case where these objects are mathematically well established.
Let us recall basically the objects of these Calculi.
- (i)
The main object of white noise analysis and quantum probability is given by the Bosonic Fock space Fock(ℍ2) associated to the Hilbert space ℍ2 of L2 maps from [0,1] into ℝ. Fock(ℍ2) is constituted of series σ = ∑hn where hn belongs to , the symmetric n-tensor product of ℍ2 such that
()The operator algebra is the algebra of annihilation and creation operator on the Fock space submitted to the canonical commutation relations()where a(s) is an elementary annihilation operator. a*(t) is an elementary creation operator. The presence of a Dirac mass leads to the same difficulties as in (1.8) and (1.9) and leads white noise analysis to consider an improvement of the Fock space (called the Hida Fock space) such that these operators act continuously on it. a(s) + a*(s) is called the white noise and can be interpreted in the measure theory. - (ii)
The main object of the Malliavin Calculus is the Lp space of the Wiener measure.
An element hn of can be realized as a symmetric map from [0,1] n into ℝ and denotes a Wiener chaos [28]. This map ψ realizes an isometry between the Fock space and the L2 of the Wiener measure. The main ingredient of the Malliavin Calculus is to take the derivative (almost surely defined!) in the direction of an element ; h ∈ ℍ2. An element is called an element of the Cameron-Martin space ℍ. This operation can be interpreted as a “nonelementary” annihilation operator on the Fock space. Through this isomorphism, a(t) + a*(t) can be interpreted as d/dtB(t), the white noise associated to the Brownian motion, which does not exist in the traditional sense because the Brownian motion is only continuous! Since there are integrations by parts associated to a derivativation along an element of the Cameron-Martin space of a cylindrical functional, this operation is closable. It is the generalization in infinite dimension of the traditional definition of Sobolev spaces on finite dimensional spaces. But in infinite dimension, we consider Gaussian measures and not Lebesgue measure, which does not exist as a measure in infinite dimension! But since there is no Sobolev imbedding in infinite dimension, functionals which belong to all the Sobolev spaces of the Malliavin Calculus (these functionals are said to be smooth in the Malliavin sense) are in general only almost surely defined!
The study of Poisson structures requests that the test functional space where this Poisson structure acts is an algebra.
- (i)
In the case of the Malliavin Calculus, there is a natural way to choose an algebra starting from the considerations of measure theory. With the intersection of all the Lp, p < ∞ is indeed an algebra through the Hoelder inequality. We consider the Wiener product on the Wiener space which is the classical product of functionals.
- (ii)
In white noise analysis, there is on the Fock space another product called the standard Wick product. The traditional product of a Wiener chaos of length n and of length m is not a chaos of length n + m by the help of the Itô formula. It is an infinite dimensional generalization of the fact that the product of two Hermite polynomials in finite dimension is not a Hermite polynomial. The classical Wick product consists to keep in the product of these two chaoses the chaoses of length n + m. Reference[29] has defined another Wick product (called the normalized Wick product), which fits well with Stratonovitch chaos. We consider now
()We consider this time multiple Stratonovitch integrals. They are the limit when k → ∞ of the classical random multiple integrals where t → Bk(t) is the polygonal approximation of the Brownian motion. The Itô-Stratonovitch integral is the classical one. The normalized Wick product of Léandre and Rogers [29] :σ1 · σ2: of σ1 and σ2 belonging to the Hida Fock space is done in order()using the Itô-Stratonovitch formula [23]. This reflects in an infinite dimensional sense the classical fact that the product of two monomials is still a monomial in a finite dimensional polynomial algebra.
Let us recall that in white noise analysis, the algebraic counterpart of the Malliavin Calculus, the main tool is the Fock space and the algebra of creation and annihilation operators on the Fock space. The Bosonic Fock space is transformed into the L2 of an infinite dimensional Gaussian measure by the help of the map Wiener chaoses. The Poisson structure (1.9) was defined by Léandre in [5] on the Hida test algebra endowed with the normalized Wick product.
The goal of this paper is to define a Lie algebroid associated to the Poisson structure (1.9) on the Nualart-Pardoux test algebra. The main remark is that the map transforms a 1-form on the Wiener space smooth in the Nualart-Pardoux sense in a generalized vector field on the Wiener space, whose theory was done by Léandre in [32, 35], and not in an ordinary vector field on the Wiener space! Classical vector field on the Wiener space are random elements of the Cameron-Martin space which belongs to all the Sobolev spaces of the Malliavin Calculus. Generalized vectorfield is where h(s) is chosen well such that we can define the anticipative Stratonovitch integral . In general, we cannot define the derivative of a functional which belongs to all the Sobolev spaces of the Malliavin Calculus along a generalized vector field. But we can do that if the functional belongs to all the Sobolev spaces in the Nualart-Pardoux sense. In this paper, since we consider smooth 1-forms in the Nualart-Pardoux sense, we can define still their interior product by a generalized vector field through our theory of anticipative Stratonovitch integral. So the formulas (1.1) and (1.2) are still true, but almost surely!
2. The Linear Stochastic Poisson Structure
We consider the set of continuous paths C([0,1]; ℝm) from [0,1] into ℝm endowed with the uniform topology. A typical path is denoted by t → B(t) = (Bi(t)), on which we consider the Brownian motion measure dP [38].
It should be tempting to represent dP by using the same procedure. We consider an orthonormal basis ei of ℍ. The law of the Brownian motion is represented by the series ∑eiNi where the Ni is a collection of independent centered one-dimensional Gaussian variables. This series does not converge in ℍ but in C([0,1]; ℝm) [45]. We refer to the textbook of Kuo [46] for the theory of infinite dimensional Gaussian measures.
If we consider the same dyadic subdivision as before, we can introduce the polygonal approximation Bn(t) of B(t). Let us consider a “nondeterministic!” map from [0,1] into ℝmt → βt which belongs to L2([0,1]; ℝm). We can consider the random ordinary integral . It can also be easily defined. In general, the limit may not exist when n tends to infinity, because the Brownian motion is only continuous. If we can pass to the limit, we say that the limit is an anticipative Stratonovitch integral. Nualart and Pardoux [30] are the first authors who have defined some anticipative Stratonovitch integrals. An appropriate theory was established by Léandre [31–36] in order to understand some Sobolev cohomology theories on the loop space. Let us recall it quickly.
Definition 2.1. The Nualart-Pardoux test algebra N.P∞− consists of functionals F whose all Nualart-Pardoux Sobolev norms of first type and second type are finite. The elements of N.P∞− are said to be smooth in the Nualart-Pardoux sense.
Let us recall that N.P∞− is an algebra [31].
Theorem 2.2. Let β be a random element of L2([0,1]; ℝm) such that all its Nualart-Pardoux Sobolev norms are finite. Then the anticipative Stratonovitch integral
Remark 2.3. Let us motivate (2.19). Let us consider the Hilbert space ℍ2 of L2 maps from [0,1] into ℝm. The define a structure of Lie algebra on ℝm*, and therefore, on ℍ2. Let us consider two functionals F1 and F2 Frechet smooth on . Their derivatives are given by kernels
3. The Stochastic Lie Algebroid
This allows us to put the following definition.
Definition 3.1. If α and β are smooth 1-forms in the Nualart-Pardoux sense, then we define
This allows us to show the following theorem.
Theorem 3.2. [·, ·] is a continuous antisymmetric bilinear application acting on the space of smooth 1-forms in the Nualart-Pardoux sense with values in the set of smooth 1-forms in the Nualart-Pardoux sense.
Proof of Theorem 3.2. We remark that
Moreover,
Theorem 3.3. [·, ·] defines a Lie bracket.
Proposition 3.4. Let one have
Proof of Theorem 3.3. Let us consider the finite dimensional Gaussian space . A 1-form is piecewise constant as well as a vector field ht. If Fn is a functional which depends on Bn only, then ∇rFn is constant on each In,r. We put
We can define πn, [·, ·] n, and according to the line of the introduction. To a 1-form smooth in the Nualart-Pardoux sense α on the total Wiener space, we consider the 1-form Πnχnα = αn on the finite dimensional Gaussian space. We get
Let us give the scheme of the proof of this last result. When we write [[α1,n, α2,n] n, α3,n] n, there are a lot of terms which will appear. All these terms will tends separately to the corresponding term in [[α1, α2], α3]. Let us treat one of them, which will lead to double anticipative Stratonovitch integral. The other terms will be treated identically. For instance will lead to double Stratonovitch integral which will tend in all the Sobolev spaces of the Malliavin Calculus to . We can consider in these expressions the term which will lead to a double stochastic integral and which will tend to . But there are two parts in and . We will consider the parts and where we take the covariant derivative of the 1-form α2 in the direction of the generalized vector field . We will show that tends to . But in these expressions there are still two parts which can be treated similarly. We will show that the expression tends to (we consider the covariant derivative of α3,n in the direction of ).
But
Therefore
Theorem 3.5. is an anchor map. This means that for all 1-form α, β which are smooth in the Nualart-Pardoux sense all functional F are smooth in the Nualart-Pardoux sense; one has the relation:
Proof of Theorem 3.5. We get by classical results in finite dimension
4. Conclusion
We can summarize that realizes a stochastic Lie algebroid acting on the space of smooth 1-forms in the Nualart-Pardoux sense on the Wiener space and functional smooth in the Nualart-Pardoux sense on the Wiener space. takes its values in the space of generalized vector fields.